A martensitic stainless steel having excellent stress corrosion resistance and a method for manufacturing the same

Abstract

This invention proposes a martensitic stainless steel with excellent stress corrosion resistance and its manufacturing method. The chemical composition of the steel, by weight percentage, includes: 0.10–0.20% C; 0.2–0.5% Si; 0.4–0.8% Mn; ≤0.030% P; ≤0.005% S; 11.5–13.0% Cr; 0.006%–0.012% Ce; 0.15%–0.35% Al; 0.08–0.15% Ta; 0.005–0.010% Zr; 0.04%–0.08% N; Ta+Zr+Al) / N: 4.0–7.5, with the balance being Fe and unavoidable impurities. The steel plate is produced through smelting, continuous casting, heating, rolling, and heat treatment. The combined effect of the elements promotes the formation of a highly dense passivation film, Ta3N5. Through optimized design of the quenching-partitioning process, the microstructure of the steel is controlled to be tempered martensite + 6-9 vol.% stable retained austenite. The steel of this invention has a yield strength ≥919 MPa, tensile strength ≥1126 MPa, elongation ≥17%, room temperature impact energy ≥94 J, and stress corrosion cracking time ≥192 h under a 650 MPa load and a ≥95℃ chloride ion concentration 10 mg / L aqueous solution environment.

Description

Technical Field

[0001] This invention belongs to the field of ferrous metal materials technology, and specifically relates to a martensitic stainless steel with excellent stress corrosion resistance and its manufacturing method. Background Technology

[0002] Stress corrosion is the most frequent and severe form of corrosion failure in stainless steel. Cr13 series martensitic stainless steel is often used to manufacture steam turbine blades, which operate in environments filled with large amounts of steam and subjected to enormous centrifugal forces. Therefore, stress corrosion becomes its primary failure mode, posing a severe challenge to the safe service of Cr13 martensitic stainless steel.

[0003] Patent CN1729306A discloses a high-strength martensitic stainless steel with excellent resistance to carbon dioxide gas corrosion and sulfide stress corrosion cracking. By specifying a particular composition, the metal microstructure is mainly composed of tempered martensite, carbides, Laves phase, or σ phase intermetallic compounds, giving the martensitic stainless steel a high strength of over 860 MPa with a strength of 0.2%, and excellent resistance to carbon dioxide gas corrosion and sulfide stress corrosion cracking. However, this technology does not solve the problem of stress corrosion resistance in martensitic stainless steel under high-temperature environments.

[0004] Patent CN109735694 A discloses a heat treatment method for low-carbon martensitic stainless steel. By adjusting the heat treatment process, the content and morphology of reverse-transformed austenite in the steel are controlled, reducing the susceptibility to wet hydrogen sulfide stress corrosion cracking. Patent CN112410687 A discloses a sulfide-resistant martensitic stainless steel material and its preparation method. By controlling the content of the main components in the material, especially the content of chromium, nickel, and nitrogen, and thus controlling the chromium equivalent, nickel equivalent, and the chromium-nickel equivalent ratio, a certain content of reverse-transformed austenite is obtained to ensure the sulfide stress corrosion cracking resistance of the stainless steel material. However, reverse-transformed austenite has poor mechanical stability and is easily transformed into deformed martensite after cold working, which exacerbates the stress corrosion susceptibility. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a martensitic stainless steel with excellent stress corrosion resistance and its manufacturing method. By adding rare earth elements Ce and Ta-Zr-Al-N elements, the composite effect promotes the formation of a highly stable and dense passivation film Ta3N5. Through optimized design of the quenching-partitioning process, 6-9 vol.% of residual austenite is uniformly distributed between the tempered martensitic laths, enabling the martensitic stainless steel to achieve a crack resistance of over 192 hours under high temperature and high humidity stress corrosion conditions.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A martensitic stainless steel with excellent resistance to stress corrosion cracking is characterized in that the chemical composition of the steel, by weight percentage, comprises: 0.10–0.20% C; 0.2–0.5% Si; 0.4–0.8% Mn; ≤0.030% P; ≤0.005% S; 11.5–13.0% Cr; 0.006%–0.012% Ce; 0.15%–0.35% Al; 0.08–0.15% Ta; 0.005–0.010% Zr; 0.04%–0.08% N; with the balance being Fe and unavoidable impurities.

[0008] Furthermore, (Ta+Zr+Al) / N: 4.0~7.5

[0009] Furthermore, the stainless steel surface has a passivation film Ta3N5, and the microstructure is tempered martensite + 6 to 9 vol.% stable retained austenite.

[0010] Furthermore, the thickness of the steel plate is 25–100 mm.

[0011] Furthermore, the stainless steel has a yield strength ≥919MPa, tensile strength ≥1126MPa, elongation ≥17%, and room temperature impact energy ≥94J.

[0012] Furthermore, the stainless steel exhibits a stress corrosion cracking time of ≥192h under a load of 650 MPa and a corrosive medium of an aqueous solution with a chloride ion concentration of 10 mg / L at ≥95℃.

[0013] The reasons for using the above-mentioned components are as follows:

[0014] C: 0.10%~0.20%

[0015] Carbon (C) is an important element for the austenitization of martensitic stainless steel, but it also promotes the formation of alloy carbides, resulting in chromium depletion and affecting the corrosion resistance of the steel. Therefore, this invention controls the C content in the steel to be within the range of 0.10–0.20%.

[0016] Si: 0.2%–0.5%

[0017] Si has a good deoxidizing effect on molten steel, but under stress corrosion conditions, Si easily combines with O to form SiO2, which accumulates at the stress crack initiation site and accelerates the crack propagation rate. Therefore, this invention controls the Si content in steel to be in the range of 0.2% to 0.5%.

[0018] Mn: 0.4%–0.8%

[0019] Mn can improve the strength and hardenability of steel, but higher Mn content increases the stress corrosion susceptibility of steel. In this invention, the Mn content is controlled at 0.4%–0.8%.

[0020] P≤0.030%

[0021] Phosphorus (P) is a harmful element in steel, easily causing cold brittleness. However, strict control of P content increases production costs. Therefore, this invention controls the P content in steel to ≤0.030%.

[0022] S≤0.005%

[0023] Sulfur (S) is also a harmful element in steel, severely impairing its corrosion resistance. Therefore, this invention requires that the sulfur content in the steel be ≤0.005%.

[0024] Cr: 11.5%–13.0%

[0025] Cr significantly enhances the passivation ability of iron and plays a decisive role in the corrosion resistance of martensitic stainless steel. However, Cr also closes the austenite phase region, promotes ferrite formation, and leads to a decrease in the strength and impact toughness of the steel. Therefore, this invention controls the Cr content in the steel to be 11.5%–13.0%.

[0026] Ce: 0.006%~0.012%

[0027] Ce, a rare earth element, has high reactivity and tends to agglomerate at grain boundaries more easily than Cr, thereby reducing the formation of Cr-depleted regions. Ce also readily combines with O to form a highly stable oxide protective film, which is beneficial for improving pitting corrosion resistance. However, adding too much Ce as a rare earth element will increase the cost of the alloy. Therefore, this invention controls the Ce content to be 0.006% to 0.012%.

[0028] Al: 0.15%–0.35%

[0029] Al can play a role in nitrogen fixation in steel. At the same time, Al tends to accumulate on the passivation film of stainless steel, enhancing the density of the passivation film and thus improving its chemical stability in pitting corrosion environment. However, excessive Al will lead to poor fluidity of molten steel. Therefore, the present invention controls the Al content to be 0.15% to 0.35%.

[0030] Ta: 0.08%~0.15%

[0031] Ta readily combines with O in steel to form various oxides, creating an oxide film that exhibits strong corrosion resistance at room temperature. However, excessive Ta addition can easily cause segregation, forming high-melting-point inclusions. Therefore, this invention controls the Ta content to be 0.08%–0.15%.

[0032] Zr: 0.005%~0.010%

[0033] Zr has a good deoxidation and desulfurization effect on molten steel, and at the same time, it can strongly inhibit the slip of dislocations, thereby delaying the propagation of stress cracks. However, Zr is a rare metal and expensive. Therefore, the Zr content is controlled at 0.005% to 0.010% in this invention.

[0034] N: 0.04%~0.08%

[0035] Nitrogen (N) improves the pitting resistance of stainless steel, but excessive N content can lead to porosity and pore defects. In this invention, the N content is controlled at 0.04% to 0.08%.

[0036] (Ta+Zr+Al) / N: 4.0~7.5

[0037] Ta preferentially combines with O in steel to form a corrosion-resistant oxide film mainly composed of Ta2O5. However, this protective film begins to decompose above 90°C. N, on the other hand, can combine with free Ta to form a new passivation film, Ta3N5. Ta3N5 exhibits higher stability in high-temperature and high-humidity environments. On one hand, Al and Zr easily accumulate in the Ta3N5 passivation film, increasing its density. On the other hand, under stress, Zr strongly inhibits dislocation slip, thereby improving the passivation film's resistance to cracking under stress corrosion conditions. However, an excessively high (Ta+Zr+Al) / N ratio will generate a large number of composite inclusions, while an excessively low ratio will not achieve the desired composite effect. Therefore, this invention controls the (Ta+Zr+Al) / N ratio to be between 4.0 and 7.5.

[0038] The second technical solution of the present invention is to provide a martensitic stainless steel with excellent stress corrosion resistance and its manufacturing method, including: smelting, continuous casting, heating, rolling and heat treatment.

[0039] smelting

[0040] The scrap steel was batched according to the above alloy element ratio, and carbon powder and ferrosilicon were added. The mixture was then smelted by EAF, and then refined by AOD and LF. During the refining process, Fe-10%Ce master alloy, tantalum bars and zirconium ferrosilicon were added. At the same time, argon gas was blown and stirred to ensure the yield of Ta and Zr. The argon flow rate was 80-90 L / min. Finally, the mixture was continuously cast into slabs.

[0041] Continuous casting

[0042] The continuous casting process adopts constant casting speed, with the casting speed controlled at 1.0 to 1.2 m / min, the superheat of the tundish controlled at 20 to 25℃, and the secondary cooling water ratio controlled at 0.10 to 0.15 L / kg, in order to increase the equiaxed crystal ratio in the center of the billet. At the same time, electromagnetic stirring is used to reduce center segregation, with a current intensity of 600 to 800 A.

[0043] Slab heating

[0044] To reduce high-temperature ferrite, the target heating temperature of the slab is controlled at 1120-1160℃, and the billet is quickly sent to the descaling machine to remove iron oxide scale after exiting the furnace.

[0045] Rolling

[0046] To avoid deformation resistance increasing during rolling and causing plate shape defects, the initial rolling temperature of the slab should not be less than 1060℃. During the rolling process, high-pressure water (≥18MPa) should be used to fully descale the slab. The final rolling temperature should be controlled at 870~890℃ to promote static recrystallization and reduce the sensitivity to grain boundary stress corrosion. The thickness of the steel plate should be 25~100mm.

[0047] Heat treatment

[0048] After rolling, the steel plates are air-cooled on a cooling bed to a surface temperature of 150–250°C and then loaded into a tempering furnace for fractionation treatment. The fractionation temperature is 320–360°C and the fractionation time is 2–3 min / mm. After that, the plates are air-cooled to room temperature to obtain tempered martensite and stable retained austenite with a content of 6–9 vol.% evenly distributed between the tempered martensite laths, forming a passivation protective film to reduce the stress corrosion sensitivity of martensitic stainless steel under high temperature conditions.

[0049] Compared with existing technologies, the beneficial effects are as follows:

[0050] 1. By adding rare earth Ce and Ta-Zr-Al-N elements, the formation of a highly stable and dense passivation film Ta3N5 is promoted, maintaining the integrity of the passivation film on the surface of martensitic stainless steel in high temperature and high humidity environment, while improving the resistance to cracking of the passivation film under stress corrosion conditions.

[0051] 2. Through optimized design of the quenching-partitioning process, the content of residual austenite in the steel is controlled within the range of 6 to 9 vol.%. The residual austenite is evenly distributed between the tempered martensite laths, forming a passivation protective film, which further reduces the stress corrosion sensitivity of martensitic stainless steel under high temperature conditions.

[0052] 3. The martensitic stainless steel of this invention has a yield strength ≥919MPa, tensile strength ≥1126MPa, elongation ≥17%, room temperature impact energy ≥94J, stress corrosion cracking time ≥192h under a corrosive medium of ≥95℃ and chloride ion concentration of 10mg / L. Detailed Implementation

[0053] The present invention will be further illustrated below through examples.

[0054] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, continuous casting, heating, rolling and heat treatment.

[0055] Slab heating

[0056] The target heating temperature of the slab is controlled at 1120-1160℃. After the billet is taken out of the furnace, it is quickly sent to the descaling machine to remove the iron oxide scale.

[0057] Rolling

[0058] The initial rolling temperature of the slab shall not be less than 1060℃. During the rolling process, high-pressure water of 18MPa or above shall be used to fully descale the slab. The final rolling temperature shall be controlled at 870-890℃.

[0059] Heat treatment

[0060] After rolling, the steel plates are air-cooled on a cooling bed to a surface temperature of 150-250°C and then loaded into a tempering furnace for batching treatment. The batching temperature is 320-360°C and the batching time is 2-3 min / mm. After that, the plates are air-cooled to room temperature.

[0061] Furthermore, after the raw materials are batched, the smelting process involves EAF melting, followed by AOD and LF refining. During the refining process, Fe-10%Ce master alloy, tantalum bars, and zirconium ferrosilicon are added, while argon gas is blown and stirred to ensure the yield of Ta and Zr. The argon flow rate is 80-90 L / min, and finally, the slab is continuously cast.

[0062] Furthermore, the continuous casting process adopts constant casting speed, with the casting speed controlled at 1.0 to 1.2 m / min, the superheat of the tundish controlled at 20 to 25°C, the secondary cooling water ratio controlled at 0.10 to 0.15 L / kg, and electromagnetic stirring is simultaneously activated with a current intensity of 600 to 800 A.

[0063] Table 1 lists the components involved in each embodiment, Table 2 lists the continuous casting process parameters of the steel in each embodiment, Table 3 lists the steel process parameters of each embodiment, and Table 4 lists the test results of the mechanical properties and stress corrosion resistance of the steel in each embodiment.

[0064] Table 1. Chemical composition (%) of steel smelting in each embodiment

[0065]

[0066] Table 2. Steel continuous casting process parameters for each embodiment.

[0067]

[0068] Table 3 Steelmaking process parameters for each embodiment

[0069]

[0070] Referring to GB / T 15970.1, the stress corrosion cracking time of the example and comparative samples was determined using the constant load stress corrosion method. The load was 650 MPa, and the corrosive medium was a chloride ion aqueous solution (Cl) at ≥95℃. -The concentration was 10 mg / L, and its mechanical properties at room temperature were tested. The results are shown in Table 4. As can be seen from Table 4, under the premise of keeping the mechanical properties basically unchanged, the stress corrosion cracking time (≥192 h) of each embodiment of the present invention is higher than that of the comparative example, indicating that the steel of the present invention has better stress corrosion resistance.

[0071] Table 4. Test results of mechanical properties and stress corrosion resistance

[0072]

[0073] As can be seen from the above, the steel of this invention, through the combined effect of adding rare earth Ce and Ta-Zr-Al-N elements, promotes the formation of a highly stable and dense passivation film Ta3N5, maintaining the integrity of the passivation film on the surface of martensitic stainless steel in high-temperature and high-humidity environments, while improving the resistance to cracking of the passivation film under stress corrosion conditions. Through optimized design of the quenching-partitioning process, the residual austenite content in the steel is controlled within the range of 6–9 vol.%, with the residual austenite uniformly distributed between the tempered martensitic laths, forming a passivation protective film, further reducing the stress corrosion sensitivity of martensitic stainless steel under high-temperature conditions. The martensitic stainless steel of this invention has a yield strength ≥919 MPa, tensile strength ≥1126 MPa, elongation ≥17%, room temperature impact energy ≥94 J, and a stress corrosion cracking time ≥192 h under a 650 MPa load and a chloride ion concentration of 10 mg / L aqueous solution at ≥95°C.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A martensitic stainless steel with excellent resistance to stress corrosion, characterized in that, The chemical composition of the steel, by weight percentage, includes: 0.10%–0.20% C; 0.2%–0.5% Si; 0.4%–0.8% Mn; ≤0.030% P; ≤0.005% S; and 11.5%–13.0% Cr. 0.006%–0.012% Ce; 0.15%–0.35% Al; 0.08%–0.15% Ta; 0.005%–0.010% Zr; 0.04%–0.08% N; The balance consists of Fe and unavoidable impurities; the production process includes: smelting, continuous casting, heating, rolling, and heat treatment, among which, Slab heating The target heating temperature of the slab is controlled at 1120-1160℃. After exiting the furnace, it is quickly sent to a descaling machine to remove the iron oxide scale. Rolling The initial rolling temperature of the slab shall not be less than 1060℃, the descaling process shall be carried out, and the final rolling temperature shall be controlled at 870~890℃. Heat treatment After rolling, the steel plates are air-cooled on the cooling bed to a surface temperature of 150-250°C and then loaded into the tempering furnace for batching treatment. The batching temperature is 320-360°C and the batching time is 2-3 min / mm. After that, the plates are taken out of the furnace and air-cooled to room temperature.

2. The martensitic stainless steel with excellent stress corrosion resistance according to claim 1, characterized in that, (Ta+Zr+Al) / N: 4.0~7.

5.

3. The martensitic stainless steel with excellent stress corrosion resistance according to claim 1, characterized in that, The stainless steel surface has a passivation film Ta3N5, and the microstructure is tempered martensite + 6 to 9 vol.% stable retained austenite.

4. The martensitic stainless steel with excellent stress corrosion resistance according to claim 1, characterized in that, The thickness of the stainless steel sheet is 25-100mm.

5. The martensitic stainless steel with excellent stress corrosion resistance according to claim 1, characterized in that, The stainless steel has a yield strength ≥919MPa, tensile strength ≥1126MPa, elongation ≥17%, and room temperature impact energy ≥94J.

6. The martensitic stainless steel with excellent stress corrosion resistance according to claim 1, characterized in that, The stainless steel exhibits a stress corrosion cracking time of ≥192h under a load of 650MPa and a corrosive medium of ≥95℃ and a chloride ion concentration of 10mg / L.

7. A method for manufacturing a martensitic stainless steel with excellent stress corrosion resistance as described in any one of claims 1 to 6, the production process comprising: Smelting, continuous casting, heating, rolling, and heat treatment, characterized in that, Slab heating The target heating temperature of the slab is controlled at 1120-1160℃. After exiting the furnace, it is quickly sent to a descaling machine to remove the iron oxide scale. Rolling The initial rolling temperature of the slab shall not be less than 1060℃, the descaling process shall be carried out, and the final rolling temperature shall be controlled at 870~890℃. Heat treatment After rolling, the steel plates are air-cooled on a cooling bed to a surface temperature of 150-250°C and then loaded into a tempering furnace for batching treatment. The batching temperature is 320-360°C and the batching time is 2-3 min / mm. After that, the plates are air-cooled to room temperature.

8. The method for manufacturing a martensitic stainless steel with excellent stress corrosion resistance according to claim 7, wherein the smelting process involves batching and EAF melting, followed by AOD and LF refining. During the refining process, Fe-10%Ce master alloy, tantalum strips, and zirconium ferrosilicon are added, while argon gas is blown and stirred to ensure the yield of Ta and Zr. The argon flow rate is 80-90 L / min, and finally, the slab is continuously cast.

9. The method for manufacturing a martensitic stainless steel with excellent stress corrosion resistance according to claim 7, wherein the continuous casting process adopts constant casting speed, the casting speed is controlled at 1.0 to 1.2 m / min, the superheat of the tundish is controlled at 20 to 25°C, the secondary cooling water ratio is controlled at 0.10 to 0.15 L / kg, and electromagnetic stirring is simultaneously activated with a current intensity of 600 to 800 A.

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